The present invention relates to a polymer composition and more particularly to a novel polymer composition which is superior in processability and which exhibits an extremely superior electroconductive characteristic when doped using an electron acceptor.
As polymers used for forming electroconductive polymers there are known polyacetylenes, polyparaphenylenes, polythiophenes and polypyrroles. These polymers become employable as electroconductive polymers by being doped using certain kinds of compounds. However, the electroconductive polymers thus obtained are apt to change in quality, especially electrical characteristics, in air. Further, those polymers are poor in meltability and solubility so are extremely inferior in processability. These drawbacks cause a large obstacle to their practical use. For example, as an application of such electroconductive polymers there has been proposed their application to electrodes for a secondary battery utilizing their reversible redox characteristic. In most cases, however, they are unstable physically or chemically in the electrolyte of a secondary battery. Therefore, it is impossible to expect a stable cyclability of charge and discharge which is a basic performance required for a secondary battery. Besides, electroconductive polymers are insoluble and unmeltable because their skeleton is constituted by a .pi. electron conjugated system and this point is also a serious obstacle to their practical use. As a solution to these problems it is proposed in Japanese Patent Laid-Open No. 206170/1986 to use an electroactive polymer as an electrode material for a secondary battery which polymer is obtained by doping a polymer having a 4,4'-diphenylamine structure as a repeating unit.
However, the above diphenylamine polymer is an oligomer of a low polymerization degree, lacking in mechanical strength and moldability which the polymer should possess as a high polymer. For example, when this polymer is used as an electrode material of a secondary battery, a soluble component will dissolve out with repetition of charge and discharge, so it is impossible to expect a stable cyclability.
Moreover, in order to impart mechanical strength and moldability to the above diphenylamine polymer in addition to good electrochemical characteristics, it is necessary to obtain a polymer of a higher polymerization degree (a high polymer). But it is difficult to obtain a high polymer even according to any of the processes commonly used for the preparation of polyaromatic compounds or polyheteroaromatic compounds, such as Grignard coupling, oxidative coupling, Friedel-Crafts reaction and electrolytic oxidation polymerization. Even under severer reaction conditions, not only it is impossible to expect the realization of a higher molecular weight due to an induced hetero-linkage or crosslinking reaction, but also the polymer becomes incapable of dissolving and melting with loss in processability which is one of the advantages of high polymers. As a further problem, the polymer becomes inactive electrically.
In order to eliminate the above-mentioned drawbacks of the prior art, the present inventors have previously proposed copolymers of the following general formula (I): ##STR5## wherein R.sup.1 is a hydrogen atom or a hydrocarbon residue having 1 to 20 carbon atoms; R.sup.2 is a hydrogen atom, a hydrocarbon residue having 1 to 20 carbon atoms, furyl, pyridyl, nitrophenyl, chlorophenyl, or methoxyphenyl; n and x are each an integer not smaller than 2 (Japanese Patent Application No. 143267/1987).
However, the above copolymer involves the problem that even when doped using an electron accepting compound, they do not become electroconductive sufficiently for use as various electronic materials. Particularly, in their application to battery electrodes, there have been the following problems.
1) Because of poor electroconductivity, the resistance of the electrodes themselves is large, thus making it impossible to handle a large electric current (that is, the charge and discharge current is small).
2) The internal resistance increases and there is no voltage flatness.
3) The charge and discharge capacity is small (that is, the utilization efficiency of active materials is poor).
It is widely known to incorporate carbon black or graphite into the battery electrodes in order to improve the electroconductivity of the electrodes. In this method, however, a large amount of carbon black for example must be used to attain high electroconductivity, resulting in that the amount of active materials of the electrodes becomes smaller and hence the battery capacity is reduced to a remarkable extent.